For decades, macroscopic impedance spectroscopy techniques have characterized alternating current (AC) charge transport for a variety of materials systems and devices. Subsequent modeling of this frequency dependent behavior has revealed underlying electrolytic surface reactions, doping levels of semiconductors, the properties of interfaces in organic and inorganic multilayer devices, and charge transport in percolation network systems. However, these macroscopic methods only reveal an ensemble average of the underlying contributions of individual pathways, defects, film thickness variations, electrochemical reactions, and failure mechanisms. To probe these effects with higher spatial resolution, a series of noncontact scanning probe impedance measurement techniques have been developed, such as scanning capacitance microscopy, scanning capacitance spectroscopy, and scanning impedance microscopy. These strategies sense relatively long-range electrostatic interactions between the probe and the sample with spatial resolution on the order of 50 nm.
However, such techniques can be limited. For instance, in scanning capacitance microscopy, a non-contact mode technique, long-range tip-sample interactions contribute to the overall signal, and the topography and capacitance signals are completely convolved and cannot be separated. Scanning capacitance spectroscopy, another non-contact mode approach, is likewise hindered and can require a data collect period approaching 24 hours. Scanning impedance microscopy is another non-contact technique, also providing convolved topography and capacitance signals. To deconvolved such modulations, a two-paths method is employed: in the first scan, the topography information is gathered in tapping mode; on the second pass, the tip is lifted and held at a constant distance from the sample by re-tracing the topography data. During the second pass, the phase shift signal is recorded. Of course, a 2-pass method to gather data takes twice as long. Even so, spatial resolution is limited due to long-range electrostatic interactions with a sample surface. (See, e.g., U.S. Pat. No. 6,873,163, the entirety of which is incorporated herein by reference.)
As a result, the search for improved sensitivity and optimal spatial resolution has been an ongoing concern in the art. One approach toward quantitative measurement of chemically and electrically active components is Nanoscale Impedance Microscopy (NIM), together with associated Atomic Force Microscopy (AFM) techniques. Atomic force microscopy is described generally in U.S. Pat. No. 6,642,517, the entirety of which—and, in particular, FIGS. 1-2, 4 and 6-7 and corresponding descriptions thereof and the references cited therein—is incorporated herein by reference. More specifically, conductive atomic force microscopy (cAFM) has recently proven to be an effective method for probing current flow and resistivity variations with nanometer scale spatial resolution in gold nanowires, silicon field effect transistors, individual organic molecules, conducting polymer blends, and emissive polymers. See, respectively: M. C. Hersam, A. C. F. Hoole, S. J. O'Shea, and M. E. Welland, Appl. Phys. Lett. 72, 915 (1998); P. De Wolf, W. Vandervorst, H. Smith, and N. Khalil, J. Vac. Sci. Technol. B 18, 540 (2000); A. M. Rawlett, T. J. Hopson, L. A. Nagahara, R. K. Tsui, G. K. Ramachandran, and S. M. Lindsay, Appl. Phys. Lett. 81, 3043 (2002); J. Planès, F. Houzé, P. Chrétien, and O. Schneegans, Appl. Phys. Lett. 79, 2993 (2001); and H.-N. Lin, H.-L. Lin, S.-S. Wang, L.-S. Yu, G.-Y. Perng, S.-A. Chen, and S.-H. Chen, Appl. Phys. Lett. 81, 2572 (2002). cAFM is described generally in U.S. Pat. No. 5,874,734, the entirety of which is also incorporated hereby by reference.
In NIM, a cAFM probe is placed in direct contact with a sample. Subsequently, an AC bias is applied to the sample, and the resulting magnitude and phase of the current at the driving frequency are monitored with either a commercial impedance analyzer or a lock-in amplifier (LIA). In this manner, NIM can provide 10 nm spatial resolution maps of impedance variations. To date, this technique has probed AC charge transport across individual grain boundaries, local variations in polymer film impedance, and the frequency dependent performance of nanoelectronic devices. Common to all reports on NIM, however, is the detection limit imposed by the fringe capacitance between the sample and the conductive probe/cantilever. Reports of fringe capacitance, which acts in parallel with the tip/sample junction, have ranged from 1-100 pF, as confirmed in reports for micro- and nanoscaled ferroelectric capacitors.